Summary of Work: This project applies biochemical and molecular biological approaches to the study of the complex interplay of the molecular signaling events that occur within T cells in response to engagement of the T cell antigen receptor (TCR) and co-stimulatory receptors. It is anticipated that a better understanding of the events that occur in the normal course of T cell activation will provide logical targets to probe with regard to their importance in immune pathogenicities, such as those characterized by immunosenescence in aged animals. The current focus is upon the study of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) within these signaling cascades.Of particular interest has been the TCR-proximal PTK ZAP-70, which is required for T cell activation. By comparing the signaling pathways in Jurkat T cells that are either deficient or replete for ZAP-70 we have recently discovered that ZAP-70 regulates the activity of the PTK Itk, which is involved in regulating intracellular concentrations of calcium ion, a key cellular regulator. The mechanism by which ZAP-70 regulates Itk activity is an indirect one, in that ZAP-70 phosphorylates the linker protein Lat, allowing Itk to form activating interactions with Lat and other proteins that associate with Lat as a consequence of Lat phosphorylation. These studies have helped to establish a new paradigm for signal transduction pathways in T lymphocytes. The precise mechanism of Itk activation and its mode of action in regulating calcium metabolism is still under investigation.In the course of these studies we also made the unexpected observation that signals can be sent through the TCR to downstream effector pathways in the absence of ZAP-70. This alternative signaling pathway is characterized by requiring extensive engagement of surface TCR and the production of weak/transient signals. Despite the weakness and transience of these signals, the existence of a TCR signaling pathway that can bypass ZAP-70 may be important in certain circumstances, allowing a TCR signal to be transmitted in anergized T cells. The possible importance of this pathway in mediating signals initiated in anergized T cells, in response to stimulation with activated B cells or superantigen is being further investigated - T Lymphocytes, Signal Transduction, Antigen Receptor, Jurkat, Immunosenescence, ZAP-70, Itk, Transcription Factors